During the laying of an offshore pipeline, it is sometimes necessary to ‘abandon’ the pipeline and ‘recover’ it later. Often this is due to severe weather conditions. The pipelaying vessel may still be on-site, but for many reasons, it is best to locate the pipeline fully on the sea floor until pipelaying operations can start again by recovering the pipeline from the sea floor. This operation is commonly termed an ‘abandonment and recovery system’, frequently abbreviated to ‘A&R system’.
Various A&R systems are known, starting with the use of cranes or winches, and lifting cables. For example, a lifting cable can be connected to the upper end of a pipeline and laid out until the whole length of the pipeline is safely on the sea floor. This is clearly the simplest operation for shallow depths. However, where the operation takes place in deeper water, for example (but not limited to) >1000 m, and increasingly at depths of up to 2000 m or even 3000 m, the length of the lifting cable becomes an important factor, as the weight of the cable itself becomes heavier. Thus, the load capacity of the lifting crane becomes a limiting factor as to how long a lifting cable can be used for this operation.
To overcome this, various methods have been proposed, and some used in the art. For example, EP1240451B suggests connecting one or more “light elongate members” to the end of the pipeline to reduce the tension at sea level of the ‘sealine’ before a winch is used to lower the sealine. In this way, it apparently becomes possible to employ a winch whose maximum load bearing capacity is substantially less than the maximum tension under which the vessel is designed to lay the pipeline.
EP1467944B proposes a lifting device involving first and second lifting structures which are open and closed at various times, and which allow the non-used part of a lifting cable to be stored in a non-tensioned manner on the vessel. In this way, EP1467944B particularly states that it is no longer necessary to employ an additional winch as an abandonment and recovery system.
WO2010/081847A1 discloses a method and apparatus for supporting a load involving a lifting block and associated first and second connection elements and first and second lifting devices able to act in unison to dispose the load at a required position from a floating vessel. The load could be for positioning an unlaid end portion of a pipe that is being laid on the sea floor or for recovering the end of a pipeline from the sea floor.
In our U.S. Pat. No. 5,421,674, we have proposed a device for laying flexible tubular conduits such as a pipeline on an ocean floor from a floating laying support such as a pipe laying vessel, the device comprising a recoverable flexible tubular handling conduit having sufficient mechanical strength to withstand a combined action of a highest internal pressure which said flexible tubular conduit to be laid will be subjected to, and an axial tensile force corresponding to an apparent weight in water of a length of said flexible tubular handling conduit which is suspended between said floating support and said ocean floor, and, to withstand isolated action of axial tensile force corresponding to an apparent weight in water of a length of said flexible tubular conduit to be laid suspended between said floating support and said ocean floor by said flexible conduit. The present invention is a surprising improvement of the invention described in U.S. Pat. No. 5,421,674.